## Inertial Measurement Units

Inertiai measurement units consist of a platform together with gyros and accelerometers on it. The platform is usually attached to the vehicle by gimbals. Most frequently there are three gyros and three accelerometers that have mutually orthogonal sensing axes, two parallel to the reference plane of the platform and one perpendicular to it. In a frequently used arrangement, the output from the gyros is used to maintain the platform by servo controls in a constant orientation relative to...

## Orbits and Trajectories in an Inverse Square Field

The principal forces that determine the path of a spacecraft are normally gravitation and thrust. In comparison, forces such as atmospheric drag or solar radiation pressure are small and can often be neglected. In this chapter the motion of space vehicles will be considered when gravity is the only force present, that is, at times when the vehicle is coasting without thrust or other forces acting on it. In the vicinity of a planet or other astronomical body, the gravitational field is composed...

## Solar Radiation Pressure

Solar radiation can exert an appreciable force and torque on a spacecraft. For instance, at an altitude of500 km in the earth's atmosphere, the effects of solar radiation on an orbiting spacecraft are of the same order of magnitude as the atmospheric drag. At higher altitudes, the solar radiation pressure predominates. The radiation pressure decreases with the inverse square of the distance from the sun. (Solar radiation needs to be distinguished from the solar wind, which is the name given to...

## 1

Figure 6.11 Hypothetical arrangement, with redundancy, of 24 attitude control and station-keeping thrusters. xu x2, x3 principal axes of spacecraft. closed would probably cause catastrophic impairments. A typical redundant propellant feed system is illustrated in Fig. 4.32. An attitude control feed system that would command the propellant flow to be proportional to demand would result in extremely small valves and thrusters and, as a consequence, would not be reliable. The fluid passages would...

## Atmospheric Entry

Reentry into the earth's atmosphere or entry into a planetary atmosphere calls for an understanding of the aerodynamic forces and moments on the vehicle and of the heat transferred to it. The flight paths are determined by gravity, drag, and lift. The lift, and to a lesser extent the drag, can be modulated by changes of the angle of attack, either by small thrusters or by movable aerodynamic surfaces. Also, the trajectories do not need to be restricted to a plane by banking the vehicle, without...

## Thermal Control Devices

As mentioned in Sect. 7.2, passive means of spacecraft temperature control can be obtained by selecting for the exterior surfaces of the spacecraft coatings or tapes that have the appropriate absorptivity and emissivity. Surfaces that are interior to the spacecraft are also treated in this manner. Black paint is applied to electronic boxes and other devices when electric-to-thermal dissipation is significant. An example is the digital module shown in Fig. 7.14. Although some of the generated...

## Thrust

In defining the thrust of a rocket motor as a force acting on the vehicle, one needs a definition that characterizes, to the extent possible, the motor performance in isolation from the other parts of the vehicle. When operating in the vacuum of space, the thrust is independent of the exterior of the vehicle. However, when operating in the lower atmosphere there will unavoidably be some interaction between the rocket plume and the air flow at the base of the vehicle. As is discussed further in...

## Lamberts Theorem

Lambert (1728-1777) established the surprising fact that in a central inverse-square force field the flight time between two points (fixed, as before, in the nonrotating reference frame attached to the center of attraction) is proportional to the Kepler orbit period, with a proportionality constant that depends only on the straight line distance between the points and the sum of the distances of the points from the center of attraction. Figure 5.2 illustrates the theorem and indicates the...

## Thermal Conduction

Heat transfer among spacecraft components is in part by radiation and in part by conduction. (Convective heat transfer, as it applies to rocket motors, is discussed in Sect. 4.14.) The reader is likely to be already familiar with the fundamentals of thermal conduction. Much material is readily available in several standard texts, such as the one by Carslaw and Jaeger 7 , The present section therefore merely lists some basic relations and adds some comments that apply to spacecraft. For an...

## Rate and Integrating Gyroscopes

Most spacecraft are equipped with gyroscopes of several different types. In particular, gyroscopes can be used for the determination of the orientation in inertial space of the chosen spacecraft reference axes and for the corresponding rates of change. Modern gyroscopes, either of the mechanical type or laser-ring gyros, are the product of a long development and have reached unprecedented precision. A large body of technical literature exists, much of it applicable to space technology. The...

## Euler Angles and Transformations of Coordinates

To describe the orientation of a three-dimensional Cartesian coordinate system with respect to another such system, in general three angles are needed. These then can serve as generalized coordinates. Different choices are possible, but choices usually most convenient in applications to spinning rigid bodies, and therefore also to space vehicles, are the Euler angles (p, 0, jr. To define them, we consider the Cartesian coordinate systems (Xi, X2, X3) and (Xi, x2, x t) as illustrated in Fig....

## Info

Figure 7.5 Monochromatic absorptivity emissivity at ambient temperature at beginning of life for three typical spacecraft surfaces. From Ref. 4, Agrawal, B. N., Design of Geosynchronous Spacecraft, Prentice-Hall. Copyright 1986 Prentice Hall, Inc. By permission. space technology. White and black paints are used on external surfaces of spacecraft. Electronic boxes are usually painted black to promote radiative transfer among the various components interior to the spacecraft, thereby equalizing...

## J J [x xf dm J m

I , h, h are called the principal moments of inertia. (A single subscript is used to distinguish them from the moments of inertia in the general case.) It follows easily that i I2 k2, I3 X3 (6.11) If two of the eigenvalues and therefore also two of the principal moments of inertia are equal, the eigenvector associated with the third eigenvalue determines one of the principal axes. The other two principal axes are perpendicular to this axis and to each other but otherwise can be chosen...

## Example The YoYo Despin Mechanism

Both in orbit and during orbit insertion, spacecraft and upper stage vehicles are frequently provided with spin to ensure attitude stability. At other times it may be necessary to terminate the spin permanently or at least temporarily, for example, to reorient the vehicle prior to a propellant burn. Despinn-ing can be accomplished by firing small retro-rockets mounted on the vehicle such as to produce a torque counter to the direction of spin. Alternatively, despin can be obtained by initiating...

## Gravity Gradient Effect

If, over the space occupied by a space vehicle, the external gravitational field were perfectly uniform, the resultant of the forces on the various parts of the vehicle would be simply the force obtained by placing the entire mass at the vehicle's center of mass. In most applications, the assumption that the gravitational field is uniform over the extent of the space vehicle is satisfied to very high precision. Exceptions, referred to as gravity gradient effects, occur only when the dimensions...

## Thrust Vector Control With Thrust Chamber

Figure 4.23 Nodes for pogo oscillation analysis. (Adapted from Oppenheim and Rubin 11 .) Figure 4.23 Nodes for pogo oscillation analysis. (Adapted from Oppenheim and Rubin 11 .) The analysis starts by defining as discrete nodes all the components that are likely to play a role (Fig. 4.23). These include the thrust chambers, propellant pumps, ducts, their junctions, bellows, tanks and their outlets, hydraulic accumulators, and the principal vehicle structural elements. Each node will receive...

## Reaction Wheels and Gimbaled Momentum Wheels

Among the devices for attitude control, other than thrusters, are reaction wheels (Fig. 6.13). The wheels are driven by electric motors in either direction and are capable of high rotational speeds. They are supported by Figure 6.13 Schematic of reaction wheels and magnetic desaturation solenoids, (a) Assembly of three orthogonal wheels and their desaturation coils (b) rodlike desaturation solenoid with ferrite core. L, angular momentum of spacecraft and wheels Bg, earth magnetic field. Figure...

## Sidereal Time

Similarly to UT and GMST, sidereal time (Latin sidus star) is based on astronomical determinations. It uses the vernal equinox as a reference point. A sidereal day is therefore the time interval between two successive passages of the vernal equinox across the observer's meridian. Sidereal time is internally consistent with the geocentric equatorial reference frame discussed in Sect. 1.1.2. Because of variations in the rate of rotation of the earth, sidereal time is not exactly uniform. However,...

## View Factors

As a preliminary to the radiative part of the calculation, view factors F,-y (also called configuration factors) of pairs ( , j) of nodes are introduced. They are purely geometrical quantities, depending only on the node surfaces S, and Sj and their relative configuration. The nodes, in addition to each having constant temperature, absorptivity, and emissivity, are defined such that each node either has a direct, uninterrupted ray path or none to and from each other node, including itself. The...

## Jet Damping

Rocket motor gas, as it streams through the motor case and the nozzle, generally has a damping effect on spacecraft motions. The phenomenon is referred to as jet damping 9 , As in aircraft, it can dampen pitch and yaw Figure 6.17 Schematic of a spinning spacecraft with nutation damping wheel and horizon sensor (wheel axis parallel to xY principal axis). motions. More significant, however, is the jet damping of spinning vehicles, because it counteracts the precessional motion by decreasing the...

## Problems

(1) Consider the nozzle represented in Fig. 4.35. An axisymmetric bell nozzle with zero divergence angle and the dimensions (in mm) indicated in the figure is assumed. There is zero ambient pressure at the nozzle exit. The calculation can be based on the approximation of a steady, one-dimensional, isentropic flow of a thermally and calorically perfect gas with an average ratio of 1.30 of the specific heats and a gas constant of 330 m2 (s2 K). The stagnation temperature can be taken equal to the...

## W2wi v2 v 2 Vpi Vpi v[i bnplHvpi vy1

I 4(Vpi Vi)((Vpi Vi) - 1) 1 + b npi)2(Vpi - V)4 1 (3.57) Energy is therefore gained by the spacecraft if and only if > V . The result also shows that for a given velocity ratio and impact parameter, the gain (or loss) is larger for massive planets. Comparing, for instance, Earth (mean equatorial radius 6378 km, mean orbital velocity 29.78 km s, mass 5.976 1024 kg) and Mars (3402 km, 24.13 km s, 6.418 1023 kg) and assuming the spacecraft's heliocentric velocity to be one-half the planet's...

## Universal Time UT and Greenwich Mean Solar Time GMST

In astrodynamics, for both historical and practical reasons, Universal Time is most often used. Closely related to it by a mathematical formula is the Greenwich Mean Solar Time. These systems of timekeeping are derived from observations of the sun's crossing the Greenwich meridian at noon. A solar day is the time interval between two successive crossings by the sun of the observer's meridian. Primarily because of the eccentricity of the earth's orbit about the sun, the length of the solar day...

## Attitude Control

The need for close control of the attitude i.e., orientation in space relative to some frame to be defined in each case of spacecraft follows from such requirements as the need to point antennas and sensors toward the earth or other astronomical objects. The required accuracy greatly depends on the particular application. It can be relatively low for broadcasting satellites. In some scientific applications the required accuracy of the attitude control may be 1 arc minute or less. Attitude...